The aromatic/antiaromatic characteristics of B-N and P-N analogues of benzene and cyclobutadiene have been studied using quantum chemical methods. We use established parameters such as nucleus-independent chemical shifts, charge density at the ring critical point, and stabilization energies to quantify the nature of interactions in these molecular systems. B3N3H6 and N3P3F6 resemble benzene in being aromatic, albeit to a lesser extent, while B2N2H4 and N2P2F4 are found to be aromatic, opposite to that for cyclobutadiene. A σ-π separation analysis has been performed to critically examine the contributions from the π electrons compared to that from the σ backbone. The structural aspects in the weak interaction limits such as the H-bonded cyclic trimers of HX (X = F, Cl, and Br) have also been investigated. Even in such weak interaction limits, these cyclic systems are found to be substantially stable. These H-bonded systems exhibit nonlocal polarizations across the full-perimeter of the ring that lead to aromaticity. We propose the term "H-bonded aromaticity" for such closed-loop weakly delocalized systems. This new formalism of aromaticity has the potential to explain structures and properties in supramolecular systems.

ABSTRACT:The endohedral fullerene CH 4 @C 84 has been studied using density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2). In addition to the structure with a COH bond of CH 4 in a tetrahedral pocket conformation, we find an alternative minimum, very close in energy (6.3-9.5 kJ/mol higher according to the level of theory), with the methane inverted, which we call the antipocket conformation. Computed IR spectra are reported for CH 4 @C 84 and also for the reference system CH 4 @C 60 . The calculated vibrational levels, in a harmonic approximation, reveal close-lying translational, librational, and shell-vibrational modes. The results are also presented for the isoelectronic species NH 4 ϩ @C 60 .

A series of 6-styryl-2,4-diphenylpyrylium salts exhibiting dual fluorescence has been investigated by fluorescence up-conversion in conjunction with quantum chemical calculations. The short-wavelength emission is due to an excited state localized on the pyrylium fragment and the long-wavelength emission arises from a charge-transfer state delocalized over the whole molecule. The two fluorescing states do not exhibit a precursor-successor relationship. The rise time of the short-wavelength fluorescence is smaller than 200 fs, and that of the long-wavelength emission depends on the electron-donating property of the styryl group substituent. The rise is almost prompt with the weaker donors but is slower, wavelength and viscosity dependent with the strongest electron-donating group. A model involving a S 2 /S 1 conical intersection is proposed to account for these observations.